This invention generally relates to measurement devices, and more particularly, an adjustable gage for accurately measuring the diameter of a cylindrical surface of a body such as a brake drum.
The problem which the present invention overcomes is best described in connection with FIG. 1, wherein a standard brake drum is designated by the reference numeral 10. The brake drum has an inner circular rim 12 and an internal cylindrical surface 14. The cylindrical surface 14 has an outer edge 16, proximate to the rim 12, and an inner edge 18. As is well known, during maintenance of such brake drums, the diameter of the cylindrical surface 14 is measured at a few points between the outer edge 16 and the inner edge 18. For proper operation of the brake drum 10, the cylindrical surface 14 must be smooth and substantially uniform in diameter along the axial surface 14 to assure proper contact with the brake shoes which abut against and engage the cylindrical surface 14.
FIG. 1 illustrates a known gage 20 of the type which has been used to measure an inside diameter of a brake drum.
The gage 20 has a bar 22 at one end of which there is secured a fixed carrier 24 which supports a fixed arm or jaw 26. The bar 22 also supports a slidably movable carrier 28 which supports a movable arm or jaw 30. At the extreme free end of the arm 26, there is provided a pointed anvil 32 which is directed or projects in a direction away from the movable arm 30, while the movable arm 30 supports a pointed anvil 34 which extends or is directed away from the arm 26. By placing the anvils 32, 34 within the cylindrical surface 14 and separating the carriers 24, 28, the anvils 32, 34 come into contact with the cylindrical surface 14. The movable carrier 28, which also contains a measurement module, provides a reading of the spacing between the two pointed anvils 32, 34. However, with a prior art gage of this type, there is frequently a problem in obtaining accurate measurements because there is no precise way to determine whether the anvils 32, 34 are on diametrically opposite ends of a plane which is transverse or normal to the axial direction or whether one of the anvils 32, 34 is further in or further out than the other anvil, thereby artificially providing a greater reading than the actual value of the true diameter. While skilled mechanics can sometimes position the scale relatively accurately, by eye, there is still no assurance that the readings are precise. To those that are not as experienced, substantially incorrect readings can result from the use of the prior art gage. Such incorrect measurements, in turn, may result in insufficient or excessive quantities of metal being cut from the cylindrical surface 14 and this, in turn, may deteriorate the operation of the drum.